The present invention relates to the detection and measurement of the concentration of constituents of a solution or suspension using radiation, preferably near-infrared radiation. More particularly, methods have been developed for the noninvasive measurement of the concentration of constituents such as hemoglobin and its variants and derivatives, glucose, cholesterol and its combined forms, drugs of abuse, and other analytes of clinical and diagnostic significance. Because these methods do not require the withdrawal of blood in order to perform these measurements, they are particularly suitable for home testing of glucose levels in diabetics and of urea or creatinine levels in patients undergoing home dialysis. The present invention provides a method of calibrating these measurements to obtain an absolute concentration without the requirement of obtaining extensive calibration data for each subject.
In addition to home testing, development of noninvasive clinical testing procedures has become an important goal, due to the widespread fear of AIDS and other diseases, such as hepatitis, which can be spread through the use of invasive procedures.
In the published research, a major issue in the in vivo quantification of blood analyte concentrations is the problem of how to take the signals generated by the apparatus and create from those signals an absolute value for the constituent concentration of interest. Current methods for the evaluation of concentration levels involve conversion of the signals to an estimated constituent concentration by some arbitrary algorithm using values generated by a contemporaneous set of invasive measurements from appropriately generated samples of blood or tissue. If the concentrations estimated by the converted signals and the concentrations estimated by the invasive measurements are highly correlated, then the correlation thus found is accepted as a xe2x80x9ccalibration curvexe2x80x9d for the constituent of interest. However, the calibration curve thus generated is not necessarily valid over a wider range of subjects or physiological conditions than the range used to generate the curve.
In cases where only a relative trend in the data is of interest an accurate calibration is less critical and the foregoing method is adequate. However, in many cases, either calibration data are unavailable or a more accurate estimate of the constituent concentration is required. For these cases, a calibration method applicable to all subjects under all conditions is desirable.
A number of related publications suggest the use of water as an internal standard. Since water is an absorber in the near infra-red, the general approach is to measure the optical effect of water and to compare it with the optical activity of the constituent of interest. For example, Matcher et al. (Phys. Med. Biol., 38, 177, 1993) discusses the use of certain features of the water absorption spectrum to estimate the xe2x80x9cdifferential path lengthxe2x80x9d traveled by radiation in a scattering medium which includes water. However, their calculation for the concentration of water in the tissue studied (the human forearm) varies by approximately 12% around the mean value. Other publications (Documenta Geigy, 7th edition, 1970) indicate that, depending on the tissue of interest, water concentrations can vary between 60% and 90%.
Jobsis (U.S. Pat. No. 4,805,623) describes a method in which an unknown concentration is estimated using the presence in the sample of an absorber having a known concentration. However, in the Jobsis disclosure, the absorber of known concentration is water in tissue. Jobsis states that the variability is about 15%. Thus the concentration of water is subject to the same lack of constancy as in the disclosures by Matcher et al. Jobsis does not discuss the use of any water concentration having a level sufficiently constant to employ as a universal calibration or reference level. In fact, Jobsis states that xe2x80x9cthe practice of the present invention depends strongly on the development of either a means of translating the results in terms of accepted standards, such as spectrophotometric data in clear solutions, or on the de novo development of an extensive data base where accepted standards are not relevant, i.e., in heterogeneous systems such as the brain.xe2x80x9d
Pologe (U.S. Pat. No. 5,297,548) discloses the use of simultaneous measurement on a common optical path using pulsatile signals to determine the relative amounts of the dominant absorbers: water, deoxyhemoglobin, and oxyhemoglobin. Pologe does not indicate the possible use of such an apparatus to generate a universal calibration method applicable across multiple subjects. In fact, Pologe indicates that calibration of such an apparatus is intended to be performed empirically.
Other workers, such as Carim et al. (U.S. Pat. No. 5,553,615) and Kuestner (U.S. Pat. No. 5,377, 674), also disclose the use of optical measurements for noninvasive analysis in which one or more detectors are sensitive to wavelengths in which water is the primary absorbing species. However, neither of these disclosures attempts to create a universal calibration or reference level.
As the above discussion suggests, the difficulty of in vivo calibration problem results from a combination of two factors. First, the physical pathlength over which any absorber is present in the tissue or blood is unknown and varies from person-to-person. Second, the intense scattering in tissue and its variation from person-to-person causes the unknown pathlength to be multiplied by an unknown factor that varies with wavelength as well as with subject. A successful solution to this problem requires consideration of both of these issues.
Several patents from the laboratory of the present inventor disclose various procedures which can assist in diminishing some sources of variability and provide better precision. These include U.S. Pat. No. 5,334,287, which describes the basic procedure now known as Kromoscopy, and U.S. Pat. Nos. 5,434,412, 5,424,545, 5,818,048 and 5,672,875, all of which describe improvements and variants on the basic Kromscopic system and methods. The disclosures of all the above-referenced patents are incorporated herein by reference. While many of these patents relate to methods of improving sensitivity and precision of the assays, the biological system is so complex that additional modifications and processes are helpful.
The method of the present invention makes use of the physiological fact that the kidneys and their associated regulatory systems maintain a virtually constant water concentration in the blood. These regulatory systems maintain the osmotic pressure difference across the filtration systems of the kidney at a stable level and thereby provide the renal system with maximal control over the critical function of solute filtration.
As a result of this regulation, the water concentration in the blood, as measured by a variety of techniques, varies from approximately 830-860 grams per milliliter of blood, a variation of xc2x11.8% around the average level. In contrast, the concentration of water in tissues can vary by as much as xc2x120% around the average level. This exceptionally high stability of blood water concentration can be used to calculate concentrations of other constituents in the blood.
In the present invention, this highly stable value for the concentration of water in blood is employed in a universal calibration scheme by combining optical measurements performed at two or more wavelengths in such a way as to eliminate the dependence of concentration on either the thickness of the body part, on the thickness of the absorbing regions within the body part, and on the scattering properties within the body part.
This is accomplished, in a general sense, by employing several types of normalization of the detection channel outputs. For each detection channel the output signals are scaled to fractional modulations by comparing the differential output produced by the cardiac pulse to the background output produced at diastole, as employed in pulse oximetry. In addition, the present invention contemplates additional normalization across multiple detection channels, which normalizes the fractional modulations in each detection channel to the relative amounts of water-specific information included in all the detection channels or in a specific subset of said detection channels. This normalization thereby allows expression of the detection channel responses in such a way that the water-specific information in the resultant data is maintained at a constant level, despite the effects of thickness, of scattering, or of changes in pulse magnitude. The non-water-specific information remaining in the data expressed in this way is then predominantly a function of the relative absorptivities and concentrations of other absorbing constituents in the arterial blood to that of water, and quantitative calibrations and measurements may be performed for such constituents.
In one particularly useful embodiment, readings at two or more channels or detectors containing primarily water-specific information are used separately to provide a means of normalizing the outputs from the other detection channels to achieve concentration measurements. In another embodiment, which is particularly useful when the analyte of interest has only absorption bands that overlap with those of water, the method provides normalization over the totality of the water-specific information available in all of the detection channels. This normalization method is based, in part, on the concept that the response from a series of detectors to a fractional modulation in arterial pulse can be understood and operated upon as a vector in an N dimensional space, where N is the number of simultaneous and spatially congruent detection channels. Each of these vectors can be normalized using a scalar related to the responses of the detector channels to water in vitro. These forms of calibration improve accuracy, including both sensitivity and precision, of the requisite measurements.
In all these embodiments, the initial step in obtaining this combination of measurements is to express each individual optical measurement as the ratio of the difference between the transmission maximum, produced when the arterial blood pressure is at its diastolic minimum level, and the transmission minimum, produced when the arterial blood pressure is at its systolic maximum level. Once the optical measurement has been so scaled to a fractional modulation in each detection channel, the measurements in the various optical channels can be combined in a number of ways, which will be detailed below.
Note that an accurate combination of the fractional modulations across optical channels places a stringent set of requirements on the optical measurements. First, since the condition of the sampled body part varies throughout the cardiac cycle, the measurements must be made before there is any major physical change. For example, since the pulse normally takes about 1 second, taking 25 or more measurements/second will be in short enough time intervals that the physiological changes between measurements are minor. Second, the accuracy of the result of the combination of measurements is maximized when there exists a common light path from the source or sources, through the body part, to each of the several detectors. Third, to produce accurate signal combinations, the effects of scattering on the optical measurements must be minimized by substantially eliminating light reaching the detectors with a large angle of incidence. In a preferred embodiment, light incident on the detectors at an angle greater than about 10 degrees is minimized.
When the above constraints are met, the resulting combination of measurements minimizes adverse effects caused by the concentration of water in the arterial blood, the concentration of the other constituents of the arterial blood, the absorption coefficient of water, and the absorption coefficients of the constituents. This is true because the measurements no longer depend in a substantial manner on the light scattering coefficients within the tissue, on the venous blood component, or on the thickness of the body part.
The present invention provides several alternative configurations for excluding the effect of large-angle radiation from the detectors. In one embodiment, the contribution of scattered radiation to the signal reaching the detector is minimized. The embodiment has been previously disclosed by Block et al. (U.S. Pat. No. 5,672,875) where it is disclosed that restricting the solid angles of illumination and detection substantially eliminates radiation scattered through larger angles reaching the detector(s) relative to that transmitted or scattered though smaller angles. If such apparatus is employed using the data processing means and protocol described above, then the dependence of the combined results upon scattering coefficients will be substantially eliminated.
In a second apparatus configuration, also disclosed in Block U.S. Pat. No. 5,672,875, the contribution of large-angle scattered light reaching the detectors is minimized by generating polarized radiation and allowing said polarized radiation to contact the body part. Radiation scattered through large angles will become depolarized as a consequence of such scattering, and the interposition of an analyzer in the optical path between the body part and the detectors will prevent such depolarized radiation from reaching the detectors. This second configuration may be combined with the use of restricted solid angles of illumination and detection for additional rejection of scattered radiation.
In another configuration, the contribution of radiation scattered at larger angles relative to that scattered at smaller angles or not scattered can be estimated. This estimation is performed by measuring the total radiation reaching sets of detectors with substantially identical spectral response. Each element of each set maintains a common light path with the corresponding element of another set or sets of detectors having a different spectral response characteristic. Then, the combination across spectral responses described above can be carried out on an element-by-element basis. As the geometrical positions of the corresponding elements approach the optical axis of the system, the effect of the larger angle scattered radiation in comparison with that of the directly transmitted radiation will be decreased. Appropriate processing of the multiple element data will then permit a better estimate to be made of the ratio that would be expected in the absence of scattering effects.
The invention provides a method for improving the accuracy of noninvasive, in vivo concentration measurements of a substance of interest in blood. The method has the steps of providing an illumination source that generates illumination radiation to a measurement site across a portion of the spectrum that contains absorbance bands of said substance of interest. The measurement site is illuminated with said illuminating radiation and radiation transmitted or reflected from said measurement site is detected by a detector array. In the preferred embodiment, the detector array has a plurality of detectors having distinct maximum spectral response characteristics in a different region of said portion of the spectrum used for illumination. In one embodiment, the plurality of detectors includes at least a first detector and a second detector having spectral characteristics with greater responsiveness to the absorbance bands of water than to the other constituents in the blood. Each of the detectors in said detector array provides an output signal indicative of the amount of radiation it receives in a selected time period. The method also contains the step of determining a differential value of the output signals for periods of arterial pulse by comparing values obtained during a systolic measurement cycle with values obtained during a diastolic measurement cycle. These differential values are used to generate a series of ratios of the differential output signals generated from the output signals of the non-water detectors to the differential output signals generated from the output signals of each of the water detectors. These ratios provide a means for improving accuracy of the concentration measurements. While this same set of detectors could be used in other embodiments of the invention, it is not practical if one cannot obtain a clear differentiation of the absorption bands. For example, although the absorption bands for hemoglobin, deoxyhemoglobin, and water in the spectral region from 600-1200 nm are separate enough that they can be differentiated and the first method may be used, while for glucose determination, there is too much overlap so a different method is needed. In particular, the scalar normalization is preferred as the overlap of absorption bands increases.
The methods of the invention are useful for both Kromoscopic and non-Kromoscopic (e.g., classic spectrophotometric) measuring systems but have particular advantageous qualities when used with a Kromoscopic system, which meets the conditions of simultaneity and congruence defined above. Optical measurements using detection channels with non-overlapping spectral sensitivities may also employ the methods of this invention, so long as these conditions are substantially met. Accordingly, broadband illuminating radiation or broadband detectors may be used, preferably using detectors with overlapping response. In the alternative, several different illumination sources such as LEDs may be used, with a coded response from a single detector such as is described in U.S. Pat. No. 5,424,545. Similarly, congruent illumination and sampling methods, as well as restricting the solid angle of illumination or detection, using methods such as those described in the afore-mentioned Kromoscopy patents, are useful as part of, and in conjunction with, the methods described herein. The ratios and comparisons required for these methods can be carried out using hardware such as a neural net or software run in a computer or other calculating device. These methods are particularly useful in determining concentration of constituents of blood such as hemoglobin or glucose, in vivo in human patients.
While it is theoretically possible to use the methods disclosed herein with a model system that explains all of the scattering and interferences which effect the present measurements, no such model system is needed. The present methods are equally useful with a chemometric analysis; i.e., an analysis which is not tied to a particular physical representation.
The present invention also provides for a system or apparatus to carry out the disclosed methods. In one embodiment, the apparatus has the two water-responsive channels at its heart, which provides the calibration required to achieve the desired accuracy. If the absorption bands of the analyte of interest have too much overlap with the bands of water, multiple channels which are responsive to both water and the analyte should be utilized, as indicated in the second preferred embodiment. The apparatus may use a single illumination source and a series of detectors, such as in a detector array, or detectors with different filters. Normally, a pulsatile measurement is used, with the apparatus being capable of sufficiently rapid measurements to differentiate and segregate arterial pulse effects. In addition, congruent sampling and/or illumination apparatus, particularly with restricted solid angle, may be used.
By the use of all the components of this invention, simultaneous measurements over a common optical path can be combined with the invariance of the water concentration in blood in such a way as to minimize the effect of the unknown pathlength. Thus, the constant water concentration in arterial blood, the common optical path, and the minimization of the effects of scattered radiation solve the problem of Jobsis (the lack of a translatable standard), while the constant water concentration provides a known, subject-invariant reference to obtain absolute concentrations from the relative concentrations calculated using the procedure disclosed by Pologe.
Thus, an advantage of this invention is that it provides a method for obtaining a universal calibration curve for all patients and under all conditions. This in turn permits the accurate measurement of absolute concentrations of constituents of arterial blood having absorbance in a given spectral sub-region.
Another advantage of this invention is that the method so provided can create a calibration curve for a particular constituent without requiring invasive measurements of that constituent in each subject.
Another advantage of this invention is that the method so provided can reduce measurement variability due to irregularities in cardiac pulse amplitude and the consequent pathlength variation.
Yet another advantage of this method is that it may be employed using a variety of optical configurations and in a variety of spectral sub-regions for noninvasive measurements on any of a variety of body parts, limited only by the requirement that both water and the constituent of interest have measurable absorbance in the spectral sub-region on interest, that the body part permit measurable quantities of radiation to reach the detectors, and that the cardiac pressure produce a measurable change in the radiation reaching the detectors.
Other advantages to the methods resulting from this invention will be apparent from the following detailed description of the invention.